Distillation of polymerizable compounds



April 2, 1946. E. R. ERICKSON DISTILLATION OF POLYMERIZABLE COMPOUNDSFiled Dec. 1, '1942 INE/ENTOR. Fdn/m 2?. flee/K9072 BY ATTORNEYSPatented Apr, 2, 1946 f-umrso STATES PATENT orrics 1 DISTILLATION. FPOLYMEBIZABLE COMPOUNDS Edwin R. Erickson, Niagara, Falls, N. Y.,assignor to The Mathieson Alkali Works, Inc., New York, N. Y., acorporation of Virginia Application December 1, 1942, Serial No. 461,529

5 Claims. (c1. zoz-sr) This invention is concerned with the distillationo1' materials that arev susceptible to poly;

merization upon heating. It ,is c'oncernedparticularly with thedistillation of heat-polymerizable vinyl compounds, and providesimprovements -to the end that polymerization of such materials andcompounds is inhibited at all stages of distillation.

Styrene and variously substituted styrenes, as well as otherpolymerizable vinyl compounds tend to yield considerableproportions ofpolymeric materials when they are distilled, with resultant decrease inthe yield of the desired monomeric products. It has been proposedheretofore to prevent such polymerization through the use of variousinhibitors, both volatile and non-volatile.

Non-volatile inhibitors tend to remain in the stillpot. with the resultthat in the rest of the distilhibitor for the polymerization, whichinhibitor is soluble in the compound and has substantially the samevapor pressure characteristics as the compound in the temperature rangeprevailing in the distillation system. Preferably, the inhibitor is suchthat it forms an azeotropic mixture with the compound, thereby insuringthe Presence oi the inhibitor in all phases and in all parts of thedistillaton system.

,1 have discovered further that the operation is greatly improved,especially when fractionation is involved, if the polymerlzable compoundto be'distilled is introduced (preferably continuously) into thedistillation zone by passing-in in direct heatergchange relationshipthrough at least a part of the fractionating zone. the passagepreferably being countercurrent.- .Thus, it is desirable to pass thecold charge of poiymerizable material to be distilled countercurrentlythrough the fractionation zone 01' a distillation system inindirect-heat exchanging relationship therewith,

and to introduce the materialinto the distillation system in the lowerpartthereor and preferably in the lower part of the i'ractionation zoneand just above the till-pot or boiling zone. In

- this manner the entering charge serves to cool the upper part ofthei'ractionatirig zone atthe in the distillation system.

same time that the charge-is being heated. In

such an operation, the chargecannot be super- I heated and enters thelower part of the distillation systemisay the lower part of thefractionating column), at just the right temperature to be flashed intovapor. To provide heat for this latter purpose; the lower part of thecolumn should be slightly superheated.

I have found that this method of operation,

10 with cooling of the upper part of the fractionating zone andsuperheating of the lower part. brings about marked improvement inefficiency of fractionation and appears to be equivalent in effect to aconsiderable number of theoretical plates It offers a marked improvementover operations in which the charge is first heated on the outside andintroduced into the center of the column and is also superior in resultto an operation in which the charge is introduced continuously into thestill-peter is placed initially. in the still-pot in a batchdistillation.

In operating in accordance with-my invention,

, the inhibitor selected in any particular case is such that it issoluble in the polymerizable material to be distilled and hassubstantially the same vapor pressure as the polymerizable material, sothat it is at all times present in both liquid and gaseous phases andin; allgparts of thesystem.

A indicated above, it is particularly advantageous toemploy an inhibitorthat forms anazeotropic mixture with the polymerizable material.

In selecting the inhibitor, attention must be paid to the particulartemperature range in which the distillation is to be performed. Thus, anin hibitor which may be suitable for use in the dis- .tm tlon of aparticular polymerizable material at one range of temperatures may beunsuitable in another range. For example, a. particular inhibitor mayhave nearly the same vapor pressure as a particularpolymerizable-material over one range of temperature but a substantiallydiiferent vapor pressure. as compared with the polymerizable material,at another range of temperature. Consequently, although the vaporpressurecurvesof a polymerizable material and of an inhibitor may besumciently close together over a wide range of temperature, thepossibility of a wide divergence at some points should not beoverlooked; and the operation must be conducted so that the vaporpressures of the polymerizable material and the inhibitor areapproximately the same in the temperature range prevailing during thedistillation.

As indicated above, thetsolubility oithe inhib.-

itor i the polymerizable material as wen s their relative vapor pressureis important. Thus, in the distillation oi many vinyl compounds,inhibitors such as sulfur, metals and hydroquinones are lnefl'ective,probably because of lack of solubility in the material subjected todistillation. On the other hand. catechols and resorcinols, andparticularly their alkyl derivatives are. in general, soluble in vinylcompounds and hence effective if they possess the correct vapor pressurerelationship for the temperature range at which the disv tillation isconducted. It is not necessary to employ large proportions of theinhibitors.

hibitor may be 0.1% to .05% or less, although proportions in excess of1% may be used without harmful result. a

In the practice of my invention, it is desirable to employ apparatus ofthe type illustrated in the accompanying single figure which is aschematic sectional view through a distillation apparatus,- including astill-pot, a fractionating column and a reflux column.

Referring to the drawing, it will be observed that the apparatuscomprises a charge tank to to which is supplied the crude polymerizablematerial having dissolved therein a selected inhibitor. The chargepasses into a fractionating column I I through a pipe it provided with avalve I! for control purmses and a liquid seal I 4 to prevent escape ofvapor from the column. The pipe enters the frectionating colurrm nearthe top there- .of and passes downward centrally therein to a point nearthe bottom of the fractionating column and thus discharges the materialinto a lower por- 7 tion of the"column Otherwise the fractionatingcolumn is of conventionaldesign and is provided with suitable packing l5and enclosed in a suitable jacket it which may be employed forinsulation or thermal control purposes. f v

A thermometer I! or other heat indicating device is provided at the topof the fractionating column.

v The bottom of the fractionating column is connected to a still-pot orboiler l8, provided with a conduit l9 from which bottoms may be with-Small proportions, usually 1% or less, are adequate. The proportion ofthe inmitted to accumulate in the any'substantial degree, and the smallamount of high boiling bottoms inthe still not serves to reboilthetower. Forthisp tion of relatively pure materi may be esirable to add asmall percentase oi higher boiling material to serve this re-boillnsfunction, i. a, the function of transmitting: heat from the stillpotinto the fractlonating column.

In the'foilowing detailed examples, the practice of my invention isdescribed with reference to the distillation o! crude dichlorostyrene,although the operation is also applicable to other polymeriz ablematerials, it being understood, however. that the particular inhibitoremployed mm be aslected with regard to the vapor pressure of thepolymerizsble material in the particular temmrature range prevailing inthe distillahon.

Emmplel Mixed dichlorostyrenes as well as me momsual isomers thereoftend to polymer-lac easily, es-

pecially under theinfiuence of heat. and are therefore selected forpurposes of illustrating the invention. Employing the apparatus justdescribed, mixeddichlorostyrenes were subjected to distillationemploying a variety of inhibitors.

It was found that, employing tertiary butylcate- 01101 in proportionsranging from .1 to 11%, the yield of monomeric dichlorostyrene was 93%,indicating that the vapor pressure oi the tertiary butylcatechol and itssolubility were both satisfactory, In another test, a larger proportionof tertiary butylcatechol is employed as an inhibitor. In fact, thecrude dichlorostyrene was nearly saturated with tert-butylcatechol atroom temperature. In this distillation, a yield of monomericdichlorostyrene of 98% was obtained, again indicating that thesolubility and the comparao tive volatility ofthis inhibitor under thecondidrawn and' also .provided with a 'conduit 20 through which inertgas may be introduced .into the distillation system, if desired.

1 The top of the i'ractionating column is connected by a. conduit 24 toan indirect water cooled reflux column 2|. The bottom of the refluxcolumn isprovided with a drain pipe 22 having a control valve 23 thatmay be regulated to adjust the proportion of condensed distillatereturnin t0 the fractionation column and conversely the Proportion ofsuch distillate removed from the system. The drain from the refluxcolumn, is

connected to a conventional receiver (not-shown) In starting theoperation in the apparatus just described, a small quantity of thecharge containing the inhibitor is introduced into the apparatus fromthe charge tank and flows into thestill-pot, where it is vaporized tostart the iractionatins operation and toproduce some refluxing.Thereafter, the charge. is introduced continuously and distillate andbottoms are continuously withdrawn, the effort being to minimize thetime that any given part of the charge is subjected to heatmg. Thus,'verylittle polymerizable charge isheai'ed at one time and that part isheated for only a short interval. Under these conditions of oporation,the polymerizable material is not pertions of operation were excellent.

For comparative purposes, a variety of other inhibitors were employed,and the distillation was also conducted without any inhibitor at all.

Without employing an inhibitor, the yield of monomeric dichlorostyrenewas 53%. This was' raised to only 55% when .l% of sulfur was'employed,the solubility of the sulfur being too' low and the volatility of thesulfur under the conditions of the operation being neglibile.

A metallic inhibitor, in this case aluminum. was also employed. Itsvolatility and solubility were both negligible and the yield ofmonomeric dichlorostyrene obtained through its use was only 78%;

A test conducted with .05% of diphenylamine gave a yield of 4Triphenylguanidine in proportions of .l% gave a yield of due to the factthat the melting point of this inhibitor was too high and its volatilityinsumcient.

Tests with hydroquinone and chlorohydrcquinone gave unsatisfactoryresults because they sun-pot, at all to aseaess portions ranging from60% to 82%.

, Example 2 I Two samples of dichlorostyrene containing smallproportions of alphachloroethyl dichloro- .1 to 36% ,8? yields ofbenzene and other impurities, were treated in the presence of smallamounts, 1. e., about .1.%, of tertiary butylcatechol which readilydissolved in the charge. The charge so prepared was treated bycontinuous distillation in the apparatus described hereinbefore and withinlet-rates and take-0B The other data with results thereof ratessubstantially equal. respect to the operation and the are given in thefollowing table:

It will be observed that substantially complete recovery of thedichlorostyrene, without loss of product to the polymer, was obtained inboth cases. a

.Example 3 The distilling pot was charged with 225 ml. of

50% crude dichlorostyrene containing about 1% tert. butyl' catecholandequilibrium conditions were established at a-pressure of 3 mm; :a vaportemperature of 67.5 to 68.5 C. and a charge rate approximately equal tothe take-oi! rate. A small stream ot'nitrogen bubbled through thecontents of the still-pot. Then 600 ml. of fair dichlorostyrene was putthrough the still to obtain a yield of about 650 ml. or "good qualitydichlorostyrene having N =1.5790. This is about 93% recovery of thedichlorostyrene, ,The contents of the stillpot at the end oi the runwere relatively non-viscous indicating the absence of polymericby-products.

I claim:

1. In distilling a polymerizable' compound involving treatment 01' thedistillate in a fractionation zone, the improvement which comprisesintroducing the polymerizable compound to the disfractionation in thethe presence of a small proportion of an inhibt tor for thepolymerization, which inhibitor is soluble in the compound and hassubstantially the same vapor pressure as the compound at the temperaturerange prevailing in the distillation.

2.'In distilling a polymerizable compound in volving treatment of thedistillate in a fractionation zone, the improvement which comprisesintroducing the polymerizable compound into the fractionation zonebypassing it in countercurrent indirect heat exchange relationshipthrough the fractionation zone and conducting the distil- 'lation andfractionation in the presence of a small proportion of an inhibitor forthe polymerization, which inhibitor is soluble in the compound and hassubstantially the same vapor pressure as the compound'at thetemperatures prevailing in distillation and fractionation.

3. In distilling a involving treatment of the distillate in afractionation zone, the improvement which comprises introducing thepolymerizable compound to the distillation by passing it incountercurrent indirect heat exchange relationship through thefractionation zone and conducting the distillationandpresence of a smallproportion of an inhibitor for the polymerization, which inhibitor is.soluble in the compound and has substantially the same vapor pressureasthe compound at the temperatures of operation.

4. In the distillation of monomeric dichlorstyrene compound, theimprovement which comprises conducting the distillation in the presenceof a small proportion of tertiary butyl catechol under a pressure offrom about 1-3 mm. of mercury at a temperature of from about 67-68.5 C.,

an th vapor pressures of the dichlorstyrene and the tillation by passingit in indirect heat exchange relationship through the fractionation zoneand a catechol under" these conditions being substantially the same. 5.In distilling a monomeric dichlorstyrene involving treatment of thedistillate in a fractionation zone, the improvement which comprisesintroducing the polymerizable compound to the distillation by passing itin indirect heat exchange relationship through the fractionation zoneand conducting the distillation. and fractionation in the presence of asmall proportion of tertiary butyl catechol at a temperature at whichthe dichlorstyrene. and the catechol have substantially the same vaporpressures. v I

' 1 EDWIN R.ERICKSON.

polymerizable vinyl compound

